U.S. patent number 5,459,678 [Application Number 08/169,516] was granted by the patent office on 1995-10-17 for method and calibration apparatus for calibrating computer monitors used in the printing and textile industries.
Invention is credited to Michael F. Feasey.
United States Patent |
5,459,678 |
Feasey |
October 17, 1995 |
Method and calibration apparatus for calibrating computer monitors
used in the printing and textile industries
Abstract
An apparatus for calibrating color settings of a computer
monitor to cause a proofed image, i.e., a prepress image, to
essentially match a printed image on a particular medium, allowing
the aesthetic quality of the image to be adjusted prior to
printing, thus saving time and money. Embodiments of the present
invention comprise a first set of separate red, green and blue
monitor sensors (RGB) coupled to the computer monitor to sense a
reference image and a second set of RGB ambient sensors facing
upwards to sense ambient illumination. A set of RGB digital
displays indicate numerical values representative of the computer
monitor illumination as read by the monitor sensors and adjusted by
the ambient sensors. A predefined table is used to reference the
indicated values for particular medium and these values are used to
adjust gamma values on the red green and blue color guns of the
computer monitor.
Inventors: |
Feasey; Michael F. (San
Clemente, CA) |
Family
ID: |
22616030 |
Appl.
No.: |
08/169,516 |
Filed: |
December 17, 1993 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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909109 |
Jul 2, 1992 |
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14364 |
Feb 5, 1993 |
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Current U.S.
Class: |
358/518; 358/416;
358/419; 356/416; 358/405; 358/402; 358/504; 353/122;
348/E17.004 |
Current CPC
Class: |
H04N
17/02 (20130101); H04N 1/6052 (20130101); Y10S
345/904 (20130101) |
Current International
Class: |
H04N
17/02 (20060101); H04N 1/60 (20060101); G01K
019/00 () |
Field of
Search: |
;358/504,500,527,404,405,416,419,406 ;356/402,416 ;353/122
;364/571.07 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"RasterOps CorrectColor Calibrator"; The Seybold Report on Desktop
Publishing; Jan. 17, 1991, vol. 5, No. 10..
|
Primary Examiner: Voeltz; Emanuel T.
Assistant Examiner: Shah; Kamini S.
Attorney, Agent or Firm: Freilich; Arthur Freilich,
Hornbaker & Rosen
Parent Case Text
RELATED APPLICATIONS
This is a continuation-in-part of U.S. patent applications Ser. No.
07/909,109 filed Jul. 2, 1992 now abandoned and Ser. No. 08/014,364
filed Feb. 5, 1993, now abandoned the disclosures of which are
expressly incorporated herein by reference.
Claims
I claim:
1. Apparatus for calibrating color settings of a monitor for
enabling said monitor to display a proof image which essentially
matches a corresponding image when printed on a selected standard
medium, said apparatus comprising:
a first sensor oriented to sense red, green and blue components of
infrared-filtered illumination from said monitor;
a second sensor oriented to sense red, green and blue components of
infrared-filtered ambient illumination;
means for providing a set of red, green and blue display values as
a function of said components sensed by said first and second
sensors;
a data storage table storing, for each of a plurality of standard
media, sets of predetermined red, green and blue calibration values
respectively representing white point, black point and gray balance
reference plaques;
means for causing said monitor to display said white point, black
point, and gray balance reference plaques; and
means responsive to a difference between (1) display values
produced as a consequence of said monitor displaying said reference
plaques and (2) calibration values defined by said table for said
selected standard medium, for calibrating said monitor color
settings to cause a displayed proof image to essentially match a
corresponding image when printed on said selected standard
medium.
2. The apparatus of claim 1, wherein said first sensor, said second
sensor, and said providing means are contained within a calibrator
housing detachably coupled to the monitor via suction cups.
3. The apparatus of claim 2, wherein said first sensor, said second
sensor, and said providing means are powered by a plurality of
batteries contained within said calibrator housing.
4. The apparatus of claim 3, additionally comprising an on/off
switch coupled to at least one of said suction cups wherein power
from said batteries to said first sensor, said second sensor and
said providing means is automatically switched on when said suction
cup is depressed.
5. The apparatus of claim 4, wherein each said first and second
sensors individually comprise:
a photocell, sensitive to illumination between wavelengths of 400
to 700 m.mu. corresponding to red, green and blue illumination;
an infrared filter, restricting the passage of infrared radiation
below 500 m.mu. to said photocell; and
a color filter allowing the passage of either red, green or blue
illumination.
6. The apparatus of claim 5, wherein said second sensor
additionally comprises an outer layer of plexiglass to diffuse the
ambient illumination before sensing by said second sensor.
7. The apparatus of claim 6, wherein said display values are a
function of respective components of infrared-filtered illumination
sensed by said first and second sensors, where said function is
described by the equation:
where Z is one of said display values, x is a value representative
of one of said components sensed by said second sensor and y is a
value representative of one of said components sensed by said first
sensor.
8. The apparatus of claim 1, wherein said first sensor, said second
sensor, and said providing means are contained within a calibrator
housing permanently coupled to the monitor.
9. The apparatus of claim 8, wherein the monitor is driven by a
computer and said first sensor, said second sensor and said
providing means are coupled to said computer to communicate said
display values to said computer for adjusting the monitor according
to an automated procedure.
10. The apparatus of claim 9, wherein said data storage table is
contained within memory of said computer.
11. The apparatus of claim 10, wherein said first sensor, said
second sensor, and said providing means receive power from said
computer.
12. The apparatus of claim 11, wherein each said first and second
sensors individually comprise:
a photocell, sensitive to illumination between wavelengths of 400
to 700 m.mu. corresponding to red, green and blue illumination;
an infrared filter, restricting the passage of infrared radiation
below 500 m.mu. to said photocell; and
a color filter allowing the passage of either red, green or blue
illumination.
13. The apparatus of claim 12, wherein said second sensor
additionally comprise an outer layer of plexiglass to diffuse the
ambient illumination before sensing by said second sensor.
14. The apparatus of claim 13, wherein said display values are
determined according to the equation:
where Z is one of said display values, x is a value representative
of one of said components sensed by said second sensor and y is a
value representative of one of said components sensed by said first
sensor.
15. Apparatus for calibrating settings of a monitor for enabling
said monitor to display a proof image which essentially matches a
corresponding image printed on a selected standard medium, said
apparatus comprising:
a monitor sensor coupled to the face of the monitor, wherein said
monitor sensor generates a signal in response to infrared-filtered
illumination from the monitor;
an ambient sensor capable of facing upwards to sense
infrared-filtered ambient illumination at the face of the monitor,
wherein said ambient sensor generates a signal in response to
ambient illumination;
a digital display displaying a display value as a function of said
monitor sensor and said ambient sensor;
a data storage table storing for each of a plurality of standard
media, sets of predetermined calibration values respectively
representing white point, black point and gray balance reference
plaques;
means for causing said monitor to display said white point, black
point, and gray balance reference plaques; and
means responsive to a difference between (1) display values
produced as a consequence of said monitor displaying said reference
plaques and (2) calibration values defined by said table for said
selected standard medium, for calibrating said monitor settings to
cause a displayed proof image to essentially match a corresponding
image when printed on said selected standard medium.
16. The apparatus of claim 15, wherein said monitor sensor, said
ambient sensor, and said digital display are contained within a
calibrator housing detachably coupled to the monitor via suction
cups.
17. The apparatus of claim 16, wherein said monitor sensor, said
ambient sensor, and said digital display are powered by a plurality
of batteries contained within said calibrator housing.
18. The apparatus of claim 17, additionally comprising an on/off
switch coupled to at least one of said suction cups wherein power
from said batteries to said monitor sensor, said ambient sensor and
said digital display is automatically switched on when said suction
cup is depressed.
19. The apparatus of claim 18, wherein said monitor sensor and said
ambient sensor are individually comprised of:
a photocell, sensitive to illumination between wavelengths of 400
to 700 m.mu. corresponding to red, green and blue illumination;
and
an infrared filter, restricting the passage of infrared radiation
below 500 m.mu. to said photocell.
20. The apparatus of claim 19, wherein said ambient sensor
additionally comprises an outer layer of plexiglass to diffuse the
ambient illumination before sensing by said ambient sensor.
21. The apparatus of claim 20, additionally comprising a
compensator providing a compensated signal to said digital display
as a function of signals from said monitor sensor and said ambient
sensor, wherein said function is described by the equation:
where Z is said compensated signal, x is said signal from said
ambient sensor and y is said signal from said monitor sensor.
22. The apparatus of claim 21, wherein the monitor is a black and
white monitor.
23. The apparatus of claim 21, wherein the monitor is a color
monitor.
24. A method for calibrating color settings of a monitor enabling
said monitor to display a proof image which essentially matches a
corresponding image when printed on a selected standard medium,
comprising the steps of:
generating monitor signals representing red, green and blue
components of infrared-filtered illumination from the monitor;
generating ambient signals representing red, green and blue
components of infrared-filtered ambient illumination;
displaying white point, black point and gray balance reference
plaques on said monitor;
determining red, green and blue display values as a function of
said monitor signals and said ambient signals in response to each
said plaque; and
adjusting the monitor in response to a difference between (1)
display values produced as a consequence of said monitor displaying
said reference plaques and (2) pre-defined red, green and blue
calibration values corresponding to said reference plaques and said
standard medium, to cause said displayed proof image to essentially
match a corresponding image when printed on said selected standard
medium.
Description
BACKGROUND OF THE INVENTION
The present invention relates generally to apparatus suitable for
calibrating computer monitors to reproduce color images that
essentially match said images as they will be printed on different
media including various types of papers or textiles.
With the advent of high resolution computer monitors connected to
personal computers executing desktop publishing tools, it has
become possible and desirable to use the computer monitor as a
proof prior to printing, i.e., a prepress image. By first proofing
images on a computer monitor, time and expenses can be saved.
However, various factors can make the image on the computer monitor
less useful for proofing purposes. First, computer monitors are
generally controlled by three color control signals that represent
red, green and blue (RGB) as opposed to the predominant printing
process which uses cyan, magenta, yellow, and black (CMYK).
Solutions have been offered in the prior art that provide
conversions, using tables, between RGB and CMYK representations of
an image. However, these conversions rely upon a standard computer
monitor. Unfortunately, computer monitors are not standard.
Computer monitors manufactured by different manufacturers or
processes respond differently to the same RGB signals, generally in
a nonlinear manner to each color (R, G or B). Additionally,
computer monitors generally have external controls, e.g., contrast
and brightness, that effect their output. Also, an observer's
perception of an image on a computer monitor is altered dependent
upon ambient illumination. Therefore, to be useful for proofing, a
method is required to standardize a computer monitor's output so
that a reproduced image will be useful as a color reference to an
observer.
SUMMARY OF THE INVENTION
The present invention is directed toward a method and apparatus for
calibrating a computer monitor in various ambient lighting
conditions to reproduce a color image such that said reproduced
color image, also known as a prepress image, can be used as a proof
prior to printing on various types of papers or textiles.
First, embodiments of the present invention filter infrared
radiation from a computer monitor that is to be calibrated since
such nonvisible radiation can disrupt readings that are intended to
relate to the visible spectrum. Second, a first set of RGB sensors,
also filtered from infrared radiation, are arranged to provide
signals representative of the amount of red, green or blue, that is
present on an image displayed on the monitor. Third, a second set
of RGB sensors, e.g., pointed upwards, sense the RGB values of the
ambient illumination and are used to compensate the signals from
the first set of RGB sensors to obtain values independent of the
ambient illumination. The compensated values are preferably
displayed on a set of numerical displays, for each prime color.
The numerical displays are preferably calibrated to a standard
computer monitor's output. To obtain a standard computer monitor, a
computer monitor and a reference image on paper or textile are
subjected to a standard illumination, e.g., 30 foot candles as
specified by the Illumination Engineers Society (IES) with a
transmissive color temperature of 5,000 degrees Kelvin as specified
by the American Standards Institute PH2.30 or 7,500 degrees Kelvin
as specified by the American Society for Testing Materials ASTM
D1684-61. An observer accordingly adjusts the computer monitor to
essentially match the color from a print of the reference image,
preferably utilizing a gamma adjustment to match the nonlinear
response of a printing process to that of the computer monitor. A
calibrator that embodies the present invention is then adjusted to
a desired value for the reference image.
Since, different media, e.g., paper types, respond differently to
printing, a data look-up table is preferably generated that
corresponds to different media, e.g., white paper or newsprint,
that the computer monitor is intended to represent. To generate
this table, the aforementioned process is repeated once for each
media type.
In accordance with a preferred embodiment, the computer monitor
calibrator is primarily comprised of (1) a plurality of sensors
coupled to the face of a computer monitor where each sensor is
comprised of an infrared filter, a color filter and a photocell,
(2) a plurality of sensors faced upwards to sense ambient radiation
where each sensor is comprised of an infrared filter, a color
filter and a photocell, and (3) a plurality of digital displays
that display a value representative of the computer monitor sensors
compensated by the ambient sensors.
In accordance with a further aspect of the preferred embodiment, a
predefined table is provided that lists a white point, a black
point and a gray balance for each prime color and for each defined
media type.
Other features and advantages of the present invention should
become apparent from the following description of the
presently-preferred embodiments, taken in conjunction with the
accompanying drawings, which illustrate, by way of example, the
principles of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a preferred embodiment of a calibrator
in accordance with the present invention;
FIG. 2 is a chart showing the spectral response of a cadium sulfide
photocell as used in a preferred embodiment;
FIG. 3 is a chart showing the spectral response of Wratten color
filters as used in a preferred embodiment;
FIG. 4 is a chart showing the spectral response of a CM-500N
infrared absorption filter as used in a preferred embodiment;
FIG. 5 is a front perspective view of a preferred calibrator in
accordance with the present invention;
FIG. 6 is a rear perspective view of a preferred calibrator in
accordance with the present invention;
FIG. 7 is a rear cutaway perspective view of a preferred calibrator
in accordance with the present invention;
FIG. 8 is a schematic of electronics that embody a calibrator in
accordance with the present invention;
FIG. 9 is a diagram of the calibrator in accordance with the
present invention used in conjunction with a computer monitor;
FIGS. 10(a-c) are examples of calibration plaques used for
calibration with the calibrator of the present invention;
FIGS. 11(a-c) are graphs of uncorrected, part corrected and fully
corrected red, blue and green gamma curves;
FIG. 12 is a front view of a preferred embodiment of a calibrator
with a printed calibration table;
FIG. 13 is a representation of the calibration procedure using a
preferred embodiment;
FIG. 14 is a representation of the environment for calibrating the
calibrator of the present invention.
FIG. 15 is a representation of the calibration procedure for a
standard computer monitor;
FIG. 16 is a flow chart of the calibrator calibration
procedure;
FIG. 17 is a flow chart of the computer monitor ink calibration
procedure; and
FIG. 18 is a top level flow chart showing the ability of a
calibrator of the present invention to essentially match a color
image between computer monitors and a printed image.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Throughout the following detailed description, similar reference
characters refer to similar elements in all figures of the
drawings.
With reference now to the drawings, and particularly to FIG. 1,
there is shown a block diagram of a preferred embodiment of a
calibrator 10 in accordance with the present invention. The
calibrator 10 is used to adjust a computer monitor 12 to reproduce
a color image displayed on its face so that it will essentially
match the colors that will be reproduced when said image is printed
using a prescribed printing process. Once the computer monitor's
gamma values have been adjusted to essentially match the color
linearity of the computer monitor 12 to the printing process,
images displayed on the computer monitor 12 can be adjusted to the
aesthetic criteria of an observer and reliably used as proofs, thus
saving the time and expense of printing.
Computer monitors conventionally reproduce their images using three
electron guns which scan across a phosphor-coated screen. The three
electron guns individually activate phosphors which display the
colors red, green and blue (RGB), prime colors for this display
process. However, conventional printing processes instead apply
inks to various types of paper, e.g., white paper or newsprint,
where the inks correspond to cyan, magenta, yellow and black
(CMYK). While tables can be created for converting an RGB image to
a CMYK image, use of the RGB image as a proof relies upon a
standardized RGB image. However, computer monitors made by
different manufacturers and processes may perform differently.
Additionally, the response of a monitor to RGB signals is
essentially linear in contrast to the response of media to a CMYK
printing process. Also, user-accessible adjustments, e.g., contrast
and brightness, and various ambient lighting conditions make it
unlikely that the viewed image will represent the printed image.
Embodiments of the present invention enable a user to adjust the
RGB gamma curves of a computer monitor to standardized settings
compensated for ambient lighting.
As found in FIG. 1, the calibrator 10 in accordance with the
present invention is primarily comprised of a set of RGB monitor
sensors 14 for displaying the red, green and blue and intensities
emitted from the computer monitor 12, a set of RGB ambient sensors
16 for displaying the red, green and blue intensities of ambient
illumination, a compensator 18 for individually compensating
signals from the monitor sensors 14 with signals from the ambient
sensors 16 and a set of RGB intensity displays 20 individually
indicating compensated RGB signals from the compensator 18. The RGB
monitor sensors 14 are comprised of three individual sensors, a
first red sensor 22 sensitive to red illumination, a second green
sensor 24 sensitive to green illumination and a third blue sensor
26 sensitive to blue illumination. Each sensor uses a photocell,
respectively 28, 30 and 32, to convert illumination into an
electrical signal. In a preferred embodiment, cadium sulfide
photocells are used that are sensitive to illumination throughout
the visible spectrum from 400 to 700 m.mu., i.e., red, green and
blue, as shown in FIG. 2 which shows the photocell response.
Therefore, to provide individual indications for red, green and
blue, filters are added to block wavelengths that are not desired
as shown in FIG. 3 which shows the filter response of Wratten
filters used in a preferred embodiment. A red filter 34,
predominantly passing red illumination and blocking other light, is
placed in front of the first photocell 28 making the electrical
signal from the photocell 28 indicate only red illumination.
Similarly, a green filter 36 is used in conjunction with photocell
30 and a blue filter 38 is used in conjunction with photocell 32.
However, it has been empirically determined that a color
filter-photocell combination cannot alone sufficiently determine
the RGB illuminations from a computer monitor due to the presence
of variable amounts of nonvisible, e.g., infrared, radiation.
Therefore, a single infrared filter 40 is additionally used to
block radiation outside of the visible spectrum. Alternatively,
multiple infrared filters 40 are used individually for each color
sensor, respectively 22, 24, and 26. A spectral response of the
infrared filter 40, as found using a CM-500N infrared absorption
filter in a preferred embodiment, is shown in FIG. 4.
Even when the RGB output of a computer monitor is standardized, the
perceived color may differ when the computer monitor 12 is
subjected to various amounts of ambient radiation. As opposed to
requiring a standardized operating environment, embodiments of the
present invention permit calibrating the computer monitor 12 to its
actual operating environment. This compensation is accomplished by
sensing the ambient RGB illumination and compensating the indicated
outputs of the RGB monitor sensors 14 according to the sensed
ambient RGB illumination using RGB ambient sensors 16. The RGB
ambient sensors 16, comprised of a red sensor 42, a green sensor 44
and a blue sensor 46, are each constructed in a similar manner to
the previously described monitor sensors 14 including a photocell,
a color filter and an infrared filter. However, the RGB ambient
sensors 16 are faced upwards to sense ambient illumination striking
the face of the computer monitor 12. Additionally, a plexiglass 040
opal 48 is used as an outer layer of the RGB ambient sensors 16 to
diffuse the ambient illumination before it reaches the RGB ambient
sensors 42, 44 and 46.
The compensator 18 individually adjusts each color signal from the
monitor sensors 14 according to the color signals from the ambient
sensors 16. Therefore, the signal from photocell 28, indicating red
illumination is compensated by the signal from the ambient red
sensor 42. Similarly, the signal from photocell 30 is compensated
by the signal from the ambient green sensor 44 and the signal from
photocell 32 is compensated by the signal from ambient blue sensor
46. This compensation is done for each color individually according
to the following equation:
where Z is the compensated signal, x is the signal from the ambient
sensor 16, y is the signal from the monitor sensor 14, and a and b
are empirically derived constants.
The compensated signals are converted to digital values and
indicated on the RGB intensity displays 20. The RGB intensity
displays 20 are comprised of a red display 50 displaying the
compensated red intensity from the computer monitor 12, a green
display 52 displaying the compensated green intensity from the
computer monitor 12, and a blue display 54 displaying the
compensated blue intensity from the computer monitor 12. In a
preferred embodiment, the RGB displays 20 consist of three
individual, multi-digit, LCD displays. When the computer monitor 12
is adjusted, according to a procedure described below using the
values indicated on the RGB intensity displays 20 and a predefined
calibration table, a standard computer monitor output is achieved
that permits the computer monitor 12 to be used as a proofing
device.
With reference now to FIG. 5, there is shown a front perspective
view of a calibrator 10 in accordance with an exemplary of the
invention. The calibrator 10 is preferably contained within a
rectangular housing 56, sized to fit on the face of the computer
monitor 12. The RGB intensity displays 20, respectively the red
display 50, the green display 52 and the blue display 54, are
located on the front of the housing 56 for viewing by a observer.
As shown in its normal orientation, the RGB ambient sensors 16 are
located on the top of the housing 56 and facing upwards to sense
ambient illumination. Additionally, a printed calibration table 58
is located on the front of the housing 56 and associated with the
RGB intensity displays 20 to allow coordinated use during
calibration as described below.
Embodiments of the present invention are preferably battery
powered. A plurality of batteries are encased within the housing 56
and are accessible through a battery compartment cover 60. As will
be described below, battery power is preferably automatically
applied to electronics of the calibrator 10 when the calibrator 10
is attached to the face of the computer monitor 12.
With reference now to FIG. 6, there is shown a rear perspective
view of the housing 56 of a preferred embodiment of the calibrator
10. The housing 56 is coupled to suction cups 62 and 64, mounted on
the rear of the housing 56 that are used to detachably couple the
housing 56 to the face of the computer monitor 12. The RGB monitor
sensors 14, comprised of the red sensor 22, the green sensor 24 and
the blue sensor 26, are also located on the rear of the housing and
are contained within an eye cap 66. As shown in this preferred
embodiment, a single infrared filter 40 is associated with the RGB
monitor sensors 14. Suction cups 62 and 64 and eye cap 66 are
preferably formed from rubber or an equivalent resilient material
that will not scratch the face of the computer monitor 12. The RGB
monitor sensors 14 are located within the eye cap 66 such that when
the housing is coupled to the face of the computer monitor 12 using
suction cups 62 and 64, the RGB computer monitor sensors 14 will be
located proximate of the face of the computer monitor 12, but not
in physical contact. Additionally, the eye cap 66 prevents
extraneous ambient light from reaching the RGB monitor sensors
14.
With reference now to FIG. 7, there is shown a rear cutaway
perspective view of an embodiment of the present invention showing
means to automatically provide power to the calibrator 10 when it
is detachably coupled to the computer monitor 12. Power for the
calibrator 10 is provided by a plurality of batteries 68 contained
within a battery housing 70. Access to the batteries 68 is obtained
through the battery compartment cover 60. Battery voltage is
switched to internal electronics using a normally-off, resilient
switch 72 that is coupled to one of the suction cups, element 64 in
a preferred embodiment, via a switch arm 74. The switch arm 74 is
coupled to the suction cup 64 such that when the suction cup 64 is
coupled to the computer monitor 12, the switch arm will depress and
activate the switch 72 and supply power to the internal
electronics. Conversely, when the calibrator 10 is removed from the
computer monitor 12, resilience of the suction cup 64 and the
switch 72 will cause the switch 72 to flip to its normally-off
position, removing power from the internal electronics.
With reference now to FIG. 8, there is shown a schematic of
prototype electronics contained within the housing 56 that performs
the functions previously described in association with FIG. 1. The
electronics are constructed to permit independent adjustment of the
a and b parameters for each color, i.e., red, green and blue, via
potentiometers. Once a reference calibrator 10 is adjusted via
procedures discussed below, the initial settings of the
potentiometers can be transferred to other calibrators. Following
this initial setup, fine tuning of an individual calibrator 10, to
account for variables in circuit manufacture is conducted using a
calibrated computer monitor and ambient light source or with a
calibration unit.
In the prototype electronics of FIG. 8, the batteries 68 are
comprised of 8 AA batteries generating a nominal voltage of 12
volts. This voltage is regulated by a fixed voltage regulator 76 to
generate a fixed voltage of 5 volts to power the remaining
circuitry. The remaining circuitry is divided into three sections
corresponding to each color channel, i.e., red, green and blue
channels. Each channel is comprised of four main sections, an
adjustable voltage regulator respectively, 78, 80, and 82, a
compensated light sensor network, respectively 84, 86 and 88, a
scaling network, respectively 90, 92 and 94, and the RGB digital
displays, respectively 50, 52 and 54. Since each channel
essentially performs in the same manner, only the red channel will
be discussed below.
The adjustable voltage regulator 78, comprised of a voltage
regulator 96, a potentiometer 98 and a fixed resistor 100,
generates an adjustable and isolated voltage that is coupled to the
compensated light sensor network 84 comprised of the red sensor 22,
sensing red monitor illumination, the ambient red sensor 42,
compensating the output of the red sensor 22, an A potentiometer
102, a B potentiometer 104, and fixed resistors 106 and 108. As
previously discussed, the A potentiometer 102 and the B
potentiometer 104 are used to adjust the amount that the ambient
sensor 42 compensates the red sensor 22 as read at voltage node
110. The voltage at voltage node 110 is scaled by the scaling
network 90, comprised of a transistor 112 and fixed resistors 114,
116, 118 and 120, to be compatible with the sensitivity of the
display 50 as read across the fixed resistor 120. The green and
blue channels perform in a similar manner, with some resistance
values modified to compensate for different sensitivities of the
photocells to other color ranges. As previously discussed, this
circuit is of a working prototype that embodies the present
invention. It is expected that one of ordinary skill in the art can
envision other circuitry that embody the aforementioned
equation:
all of which are considered to be within the scope of this
invention.
With reference now to FIG. 9, there is shown the use of the
calibrator 10 of the present invention in conjunction with a
computer monitor 12. The calibrator 10 is centrally mounted to the
face of a computer monitor 12. The calibration of a computer
monitor is a function of the computer monitor 12, its drive
electronics, e.g., a video card located within a computer 122, and
ambient lighting 124 within the computer monitor's environment.
Thus, calibration of the computer monitor 12 will need to be
repeated if any of these items are altered. As previously
described, the present invention automatically compensates for
ambient lighting via the RGB ambient sensors 16. An observer 126 is
located in front of the computer monitor 12 where the observer 126
can view the calibrator 10 and a calibration window 128 displayed
on the computer monitor 12. The observer 126, using a keyboard
and/or mouse controls on the computer 122, launches a calibration
program, e.g., Knoll GAMMA, from the computer 122. The calibration
program places the calibration window 128 in one corner of the face
of the computer monitor 12 for interaction with the observer 126
and places calibration plaques 130 in the center of the screen for
sensing by the calibrator 10.
The calibration plaques 130, contained within black border 131
(FIGS. 10(a-c)), include a white point plaque 132, a gray balance
plaque 134 and a black point plaque 136. These plaques are used for
establishing gamma curves for adjusting the response of the red,
green, and blue color guns of the computer monitor 12. Black border
131 prevents distortion of ambient light readings.
As shown in FIGS. 11(a-c), without calibration the gamma curve for
each color gun is linear. Unfortunately, in its uncalibrated and
linear state a color image reproduced on such a computer monitor
will not match a printed image. Thus, the gamma curve is adjusted
to correct for this condition. To provide this adjustment for each
prime color as shown in FIGS. 11(a-c), a white point and a black
point are used to adjust the end points of the gamma curve and a
gray balance is used to adjust the center point of the gamma
curves. These curves are further adjusted for a particular printing
process.
With reference now to FIGS. 12 and 13, the computer monitor
calibration procedure is now explained. In FIG. 12, a front view of
the calibrator 10 is shown including the RGB intensity displays 20,
respectively the red display 50, the green display 52 and the blue
display 54, and the calibration table 58. The calibration table 58
is predefined using procedures described below for specified
standard media, e.g., white paper or newsprint. Standard media is
intended to represent media that are commercially available from
one or more manufacturers and reproducible such that when the same
printing process is repeated on multiple samples of the standard
media, the printed results will be appear identical to an observer.
Supplemental calibration tables for additional media can be
provided depending upon a user's requirements. Although the numbers
shown in FIG. 12 correspond to those used in a prototype of the
present invention which uses RGB intensity displays that read
values that are in the range of approximately 1700 to 2000 with a
monitor brightness range of approximately 1 to 15 foot candles at
the monitor screen, these numbers are presented in FIG. 12 for
tutorial purposes only and are not intended to represent actual
values for a particular calibrator 10 and media combination.
After mounting the calibrator 10 on the computer monitor 12, the
observer 126 visually selects the medium for which the computer
monitor is to reproduce color proofs. For tutorial purposes,
newsprint is selected. This selection signifies that numerical
values for each column are used that are in line with the word
"newsprint". Thus, the values associated with this calibration are
as follows:
______________________________________ R 1926 1721 1802 G 1940 1735
1867 B 1900 1735 1867 White Black Gray Point Point Balance
______________________________________
Using controls on the calibration window 128, the white point
calibration plaque 132 is selected and displayed on the computer
monitor 12 and sensed by the calibrator 10. For the white point,
the first column from the calibration table is used. Thus, the
following settings on the red display 50, the green display 52 and
the blue display 54 are sought:
______________________________________ R 1926 G 1940 B 1900
______________________________________
To match these values, the red, green and blue color guns are
adjusted using controls in the calibration window 128. These
adjustments are saved and this process is similarly repeated for
the black point using the black point plaque 136 and then for the
gray balance using the gray balance plaque 134. Once this is
completed nine separate calibration points, associated with a
particular process or medium for which the computer monitor 12 has
been calibrated, are collectively saved in memory of the computer
122. This process may also be repeated for other predefined media
and saved for future use. With these saved gamma curves used in
conjunction with ink calibration tables described below, the
computer monitor 12 can accurately reproduce a color image that is
useful as a proof prior to printing for printing and textile
industries.
With reference now to FIG. 14, there is shown a representation of
the environment for calibrating the calibrator 10 of the present
invention. This initial calibration is done once to set up the
calibrator 10 and to define the calibration table 58 that is used
in conjunction with the calibrator 10 to calibrate other computer
monitors. A computer monitor 12 to be used as a reference, is
placed within an environment that presents a standard ambient
illumination 124 of 30 foot candles as specified by the
Illumination Engineers Society (IES), normally from a fluorescent
device, preferably with a transmissive color temperature of 5,000
degrees Kelvin as specified by the American Standards Institute
PH2.30 or alternatively 7,500 degrees Kelvin as specified by the
American Society for Testing Materials ASTM D1684-61. In front of
the computer monitor 12 a neutral gray surface 138 is used to view
reference print material 140 that is subjected to the same ambient
illumination 124. The goal of this calibration is to configure the
computer monitor 12 to accurately reproduce the color of the
reference print material 140 and thus it is significant that the
computer monitor 12 and the reference material 140 be viewed in the
same environment and in close proximity.
With reference now to FIG. 15, a paper/textile and printing process
is chosen and the reference print material 140 is placed on the
neutral gray surface 138. The reference print material 140 is
comprised of an unprinted area 142, representative of the white
point, a gray 1.0 density plaque 144, representative of the gray
balance, and a maximum print black ink density plaque 146,
representative of the black point. As previously described in
reference to the use of the calibrator, the calibration program is
launched which displays a calibration window 128 in one corner of
the face of the computer monitor 12. For generating a standard
computer monitor, the calibration program is used to match the
computer monitor's output to the reference print material 140
according to the observer 126. For the white point, the white point
plaque 132 is loaded by the calibration program to the computer
monitor 12. The white point plaque is represented by L a b values
of L100 a0 b0 or density levels of R255, G255, B255. The observer
126 uses controls in the calibration window 128 to adjust the
computer monitor 12 to match the white point plaque 132 shown on
the computer monitor 12 to the unprinted area 142 of the reference
material 140. This procedure is similarly repeated for matching the
black point plaque 136, represented by L a b values of L0 a0 b0 or
density levels of R0, G0, B0, to the maximum print black ink
density plaque 146 and the gray balance plaque 134, represented by
L a b values of L64 a0 b0 or density levels of R76, G76, B76, to
the gray 1.0 density plaque 144. Once completed, nine points
corresponding to the white point, gray balance and black point for
the colors red, green and blue have been chosen and are saved to
memory, e.g., RAM or disk, of the computer 122. These points
determine the gamma settings for this particular computer monitor
12 subjected to the present standard ambient illumination 124 that
cause the computer monitor 12 to perform as a standard computer
monitor. While determining these matches is subjective, it is a
one-time operation that can be accomplished by an observer 126 of
ordinary skill in the art.
The calibrator 10, attached to the computer monitor 12, is now used
to determine gamma values to be placed in the calibration table 58,
as read from the RGB intensity displays 20, for the particular
paper/textile and printing process of the reference print material
140. The normal computer monitor calibration procedure is now done
as previously described with the saved settings loaded to control
the calibration window 128 and saved RGB density and gamma settings
applied to the plaques 132, 134 and 136, but the values read from
the intensity displays 20 are instead recorded for entry into the
calibration table 58. This procedure is reflected in FIG. 16, a
flow chart of the calibrator calibration process. This process is
repeated for each paper/textile and printing process combination
for which the computer monitor 12 will be used as a proofing
device. The values determined by this process are stored in the
calibration table 58 for subsequent computer monitor 12
calibrations.
Additionally a one time procedure is used to determine the a and b
constants previously described in association with the equation:
Z=(b*y)-(a*x). To empirically determine these constants, the
ambient lighting 124 is varied between color temperatures of 4,550
and 5,500 degrees Kelvin and illuminations of 20 to 45 foot
candles. As previously described, the A and B potentiometers are
recursively adjusted to obtain values on the RGB intensity displays
20 which are independent of the ambient lighting when the gamma
curves are adjusted for matching the reference print material 140
and a maximum print ink density 146 and gray 1.0 density 144, when
subjected to altered ambient lighting 124.
With reference to FIG. 17, an additional one time process is the
generation of tables of settings that convert CMYK monitor color
gamut values of prepress images to that of print color gamut
values. The prepress images are generated from the conversion of
RGB image values to CMYK values, and from black and white negative
or positive digitized cyan, magenta, yellow, black (CMYK) color
separation images into color CMYK image values. This process for
the generation of tables of settings is repeated for each printing
process. The computer monitor 12 is calibrated with the calibrated
calibrator 10 using predetermined values in the calibration table
58 for a specified printing process. The selected values in the
calibration table 58 define the white point simulation of the
selected print medium, the black point selects the maximum print
ink density and color of the black and color ink combination that
produces the maximum ink density and the gray balance of the
calibration table 58 defines the neutral gray balance of the
printing process. A CMYK color chart of approximately 20,000 colors
is printed to represent the printing process. A printed image 148
of the CMYK color chart is prepared for the printing process and a
monitor display image 150 representing the values of the printed
CMYK color chart is produced with the computer 122 under control of
software, e.g., Adobe Photoshop. The computer monitor 12 with the
monitor display image 150 of the CMYK color chart and the printed
image 148 of the CMYK color chart are displayed in an area of
illumination of the 5,000 degree Kelvin at a level of illumination
of 30 foot candles, being the approved specifications as previously
defined, as provided by the ambient lighting 124. An observer 126
with knowledge of the process and CIE standard vision compares the
monitor display image 150 with that of the printed image 148 of the
CMYK color chart. Upon examining and comparing the color gamut of
the computer monitor image to that of the printed material,
adjustments are made for required cyan, magenta, yellow, red, green
and blue in the computer monitor representation such that the
computer monitor image 150 will match the printed image 148. An ink
color gamut adjustment table 152 and a printing specification
look-up table 154 of existing software tools, e.g., Adobe
Photoshop, are loaded to the computer monitor 12 and the CIE XYZ
coordinates of the printing specification look-up tables of the
cyan, magenta, yellow, red, green, blue areas of the CIE XYZ color
space are remapped applying printing specification adjustments that
include dot gain compensation. The monitor display image 150 of the
printed color chart and prepress image is thus changed to match the
printed color chart and the printed image 148. This process can
also be applied to the generation of settings to convert RGB values
into print, e.g., CMYK values.
With reference now to FIG. 18, there is shown a top level flow
chart of the use of the calibrator 10 of the present invention. The
printed image 148 represents printed images or a color chart
reflecting the color gamut of a printing process that is to be
proofed prior to printing on computer monitors 156 and 158. The
computer monitors 156 and 158 can be located in different
facilities in different areas of the world. For a user of the
computer monitor 156 to proof an image that was aesthetically
adjusted by a user of the computer monitor 158 it is desirable that
both images essentially match each other and the printed image 148.
It is to be expected that without adjustments, there will not be an
adequate match between these images. To provide this match, the
embodiments of the present invention are used as previously
described and summarized in Blocks 160, 162 and 164 that reflect
one-time operations associated with embodiments of the present
invention. In Block 160, a computer monitor 12 is adjusted to
standardize its output to a particular paper/textile and printing
process. This standard computer monitor is used to calibrate the
calibrator 10 and to predetermine values for the calibration table
58 in Block 162. In Block 164, the standard computer monitor is
used as a reference with gamut modification software to generate
conversion tables between the computer monitor's RGB display and
the particular printing process. The procedure of block 164 is
further reflected in FIG. 17, a flow chart of the printing color
gamut calibration process. In Block 166, the calibrated calibrator
10 using the predetermined values in the calibration table 58 is
used to calibrate computer monitors 156 and 158. These calibrations
are saved for each computer monitor and applied along with the
printing color gamut settings to display prepress images. As a
result of this process, the prepress images on computer monitors
156 and 158 will now represent a proof of the printed image
148.
In an alternative embodiment, the calibrator 10 can be integrated
into the computer monitor 12 rather than being detachably mounted
as previously described. As a consequence of this combination, the
calibration of the computer monitor 12 can be monitored more
frequently.
In a next alternative embodiment, an automated procedure displays
the calibration plaques and interfaces to the calibrator 10 to
directly read the compensated red, green and blue values from the
computer monitor 12 and automatically adjust the computer monitor's
gamma curves accordingly to a desired printing or multimedia
process according to the calibration table 58 stored within the
memory of the computer 122. The interface between the calibrator 10
and the computer 122 can be done via Apple Desktop BUS (ADB) port,
a wireless interface, or equivalent data interface. When the
calibrator 10 is integrated into the computer monitor 12, this
embodiment allows frequent, automated adjustments of the computer
monitor 12. With a wired interface such as the ADB port, power can
be provided directly to the calibrator 10.
Embodiments of the present invention can also be used to calibrate
black and white, monochrome computer monitors. In such an
environment, only one sensor is required, e.g., the monitor red
sensor 22 and the ambient red sensor 42. By this process, computer
monitors at local and remote sites being so calibrated and ink
tables loaded will display black and white prepress images as a
proof of how they will print.
Embodiments of the present invention can also be used as a step of
calibrating a color scanner for RGB gamma settings. By using a
color scanner to scan a color plaque and displaying the scanned
image on the calibrated computer monitor 12, the calibrator 10 can
display values that can be applied to adjust the scanner interface
or to a color correction program.
In another embodiment, the ambient sensors 16 are effectively
disabled by preventing ambient light from reaching the ambient
sensors 16 with an 040 density opal at sufficient angle in front of
the monitor sensors 14. This embodiment permits readings of the
ambient light at or where monitors are intended for use, as
described below. The monitor sensor readings at the intended area
of the face of a monitor are compared to a table or range of
suitable ambient readings such that a determination of suitability
can be made or an adjustment determined for permitting the ambient
light conditions to be modified to bring it within the range
suitable for such monitor calibration. Therefore, by such means a
quick determination of ambient light suitability can be determined
without proceeding with monitor calibration or having a monitor
present.
With reference to FIG. 18, a further embodiment is shown. In this
embodiment, an image is aesthetically adjusted by the observer 126
of the computer monitor 156. This image is saved into a document
format that also saves the printing color gamut settings of Block
164. In Block 160, the calibrated calibrator 10 uses the
predetermined values in the calibration table 58 to calibrate
computer monitors 156 and 158. These calibrations are saved for
each computer monitor. As a result of this process, the prepress
image on computer monitors 156 and 158 will now serves as proofs,
representative of the printed image 148.
Although the present invention has been described primarily for
CMYK correction for prepress applications, it should be understood
that it is also applicable for RGB monitor calibration useful for
video and multimedia applications. Those of ordinary skill in the
art will appreciate that various modifications can be made without
departing from the invention. Accordingly, the invention is defined
by the following claims.
* * * * *